Ketogenesis promotes tolerance to Pseudomonas aeruginosa pulmonary infection

Pseudomonas aeruginosa is a common cause of pulmonary infection. As a Gram-negative pathogen, it can initiate a brisk and highly destructive inflammatory response; however, most hosts become tolerant to the bacterial burden, developing chronic infection. Using a murine model of pneumonia, we demonstrate that this shift from inflammation to disease tolerance is promoted by ketogenesis. In response to pulmonary infection, ketone bodies are generated in the liver and circulate to the lungs where they impose selection for P. aeruginosa strains unable to display surface lipopolysaccharide (LPS). Such keto-adapted LPS strains fail to activate glycolysis and tissue-damaging cytokines and, instead, facilitate mitochondrial catabolism of fats and oxidative phosphorylation (OXPHOS), which maintains airway homeostasis. Within the lung, P. aeruginosa exploits the host immunometabolite itaconate to further stimulate ketogenesis. This environment enables host-P. aeruginosa coexistence, supporting both pathoadaptive changes in the bacteria and the maintenance of respiratory integrity via OXPHOS.

Immunometabolic control by Klebsiella pneumoniae

Klebsiella pneumoniae is a common Gram-negative pathogen associated with community-acquired and healthcare-associated infections. Its ability to acquire genetic elements resulted in its rapid development of resistance to virtually all antimicrobial agents. Once infection is established, K. pneumoniae is able to evade the host immune response and perhaps more importantly, undergo metabolic rewiring to optimize its ability to maintain infection. K. pneumoniae lipopolysaccharide and capsular polysaccharide are central factors in the induction and evasion of immune clearance. Less well understood is the importance of immunometabolism, the intersection between cellular metabolism and immune function, in the host response to K. pneumoniae infection. Bacterial metabolism itself is perceived as a metabolic stress to the host, altering the microenvironment at the site of infection. In this review, we will discuss the metabolic responses induced by K. pneumoniae, particularly in response to stimulation with the metabolically active bacteria versus pathogen-associated molecular patterns alone, and their implications in shaping the nature of the immune response and the infection outcome. A better understanding of the immunometabolic response to K. pneumoniae may help identify new targets for therapeutic intervention in the treatment of multidrug-resistant bacterial infections.

Staphylococcus aureus host interactions and adaptation

Invasive Staphylococcus aureus infections are common, causing high mortality, compounded by the propensity of the bacterium to develop drug resistance. S. aureus is an excellent case study of the potential for a bacterium to be commensal, colonizing, latent or disease-causing; these states defined by the interplay between S. aureus and host. This interplay is multidimensional and evolving, exemplified by the spread of S. aureus between humans and other animal reservoirs and the lack of success in vaccine development. In this Review, we examine recent advances in understanding the S. aureus-host interactions that lead to infections. We revisit the primary role of neutrophils in controlling infection, summarizing the discovery of new immune evasion molecules and the discovery of new functions ascribed to well-known virulence factors. We explore the intriguing intersection of bacterial and host metabolism, where crosstalk in both directions can influence immune responses and infection outcomes. This Review also assesses the surprising genomic plasticity of S. aureus, its dualism as a multi-mammalian species commensal and opportunistic pathogen and our developing understanding of the roles of other bacteria in shaping S. aureus colonization.

Klebsiella pneumoniae induces host metabolic stress that promotes tolerance to pulmonary infection

K. pneumoniae sequence type 258 (Kp ST258) is a major cause of healthcare-associated pneumonia. However, it remains unclear how it causes protracted courses of infection in spite of its expression of immunostimulatory lipopolysaccharide, which should activate a brisk inflammatory response and bacterial clearance. We predicted that the metabolic stress induced by the bacteria in the host cells shapes an immune response that tolerates infection. We combined in situ metabolic imaging and transcriptional analyses to demonstrate that Kp ST258 activates host glutaminolysis and fatty acid oxidation. This response creates an oxidant-rich microenvironment conducive to the accumulation of anti-inflammatory myeloid cells. In this setting, metabolically active Kp ST258 elicits a disease-tolerant immune response. The bacteria, in turn, adapt to airway oxidants by upregulating the type VI secretion system, which is highly conserved across ST258 strains worldwide. Thus, much of the global success of Kp ST258 in hospital settings can be explained by the metabolic activity provoked in the host that promotes disease tolerance.

Staphylococcus aureus adaptive evolution: Recent insights on how immune evasion, immunometabolic subversion and host genetics impact vaccine development

Despite meritorious attempts, a S. aureus vaccine that prevents infection or mitigates severity has not yet achieved efficacy endpoints in prospective, randomized clinical trials. This experience underscores the complexity of host-S. aureus interactions, which appear to be greater than many other bacterial pathogens against which successful vaccines have been developed. It is increasingly evident that S. aureus employs strategic countermeasures to evade or exploit human immune responses. From entering host cells to persist in stealthy intracellular reservoirs, to sensing the environmental milieu and leveraging bacterial or host metabolic products to reprogram host immune responses, S. aureus poses considerable challenges for the development of effective vaccines. The fact that this pathogen causes distinct types of infections and can undergo transient genetic, transcriptional or metabolic adaptations in vivo that do not occur in vitro compounds challenges in vaccine development. Notably, the metabolic versatility of both bacterial and host immune cells as they compete for available substrates within specific tissues inevitably impacts the variable repertoire of gene products that may or may not be vaccine antigens. In this respect, S. aureus has chameleon phenotypes that have alluded vaccine strategies thus far. Nonetheless, a number of recent studies have also revealed important new insights into pathogenesis vulnerabilities of S. aureus. A more detailed understanding of host protective immune defenses versus S. aureus adaptive immune evasion mechanisms may offer breakthroughs in the development of effective vaccines, but at present this goal remains a very high bar. Coupled with the recent advances in human genetics and epigenetics, newer vaccine technologies may enable such a goal. If so, future vaccines that protect against or mitigate the severity of S. aureus infections are likely to emerge at the intersection of precision and personalized medicine. For now, the development of S. aureus vaccines or alternative therapies that reduce mortality and morbidity must continue to be pursued.

Immunometabolites Drive Bacterial Adaptation to the Airway

Pseudomonas aeruginosa and Staphylococcus aureus are both opportunistic pathogens that are frequently associated with chronic lung infections. While bacterial virulence determinants are critical in initiating infection, the metabolic flexibility of these bacteria promotes their persistence in the airway. Upon infection, these pathogens induce host immunometabolic reprogramming, resulting in an airway milieu replete with immune-signaling metabolites. These metabolites are often toxic to the bacteria and create a steep selection pressure for the emergence of bacterial isolates adapted for long-term survival in the inflamed lung. In this review, we discuss the main differences in the host immunometabolic response to P. aeruginosa and S. aureus, as well as how these pathogens alter their own metabolism to adapt to airway metabolites and cause persistent lung infections.

NleB2 from enteropathogenic Escherichia coli is a novel arginine-glucose transferase effector

During infection, enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC) directly manipulate various aspects of host cell function through the translocation of type III secretion system (T3SS) effector proteins directly into the host cell. Many T3SS effector proteins are enzymes that mediate post-translational modifications of host proteins, such as the glycosyltransferase NleB1, which transfers a single N-acetylglucosamine (GlcNAc) to arginine residues, creating an Arg-GlcNAc linkage. NleB1 glycosylates death-domain containing proteins including FADD, TRADD and RIPK1 to block host cell death. The NleB1 paralogue, NleB2, is found in many EPEC and EHEC strains but to date its enzymatic activity has not been described. Using in vitro glycosylation assays combined with mass spectrometry, we found that NleB2 can utilize multiple sugar donors including UDP-glucose, UDP-GlcNAc and UDP-galactose during glycosylation of the death domain protein, RIPK1. Sugar donor competition assays demonstrated that UDP-glucose was the preferred substrate of NleB2 and peptide sequencing identified the glycosylation site within RIPK1 as Arg603, indicating that NleB2 catalyses arginine glucosylation. We also confirmed that NleB2 catalysed arginine-hexose modification of Flag-RIPK1 during infection of HEK293T cells with EPEC E2348/69. Using site-directed mutagenesis and in vitro glycosylation assays, we identified that residue Ser252 in NleB2 contributes to the specificity of this distinct catalytic activity. Substitution of Ser252 in NleB2 to Gly, or substitution of the corresponding Gly255 in NleB1 to Ser switches sugar donor preference between UDP-GlcNAc and UDP-glucose. However, this switch did not affect the ability of the NleB variants to inhibit inflammatory or cell death signalling during HeLa cell transfection or EPEC infection. NleB2 is thus the first identified bacterial Arg-glucose transferase that, similar to the NleB1 Arg-GlcNAc transferase, inhibits host protein function by arginine glycosylation.

Bacterial gut pathogens including enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC), manipulate host cell function by using a type III secretion system to inject ‘effector’ proteins directly into the host cell cytoplasm. We and others have shown that many of these effectors are novel enzymes, including NleB1, which transfers a single N-acetylglucosamine (GlcNAc) sugar to arginine residues, mediating Arg-GlcNAc glycosylation. Here, we found that a close homologue of NleB1 that is also present in EPEC and EHEC termed NleB2, uses a different sugar during glycosylation. We demonstrated that in contrast to NleB1, the preferred nucleotide-sugar substrate of NleB2 is UDP-glucose and we identified the amino acid residue within NleB2 that dictates this unique catalytic activity. Substitution of this residue in NleB2 and NleB1 switches the sugar donor usage of these enzymes but does not affect their ability to inhibit host cell signalling. Thus, NleB2 is the first identified bacterial arginine-glucose transferase, an activity which has previously only been described in plants and algae.

An acquired acyltransferase promotes Klebsiella pneumoniae ST258 respiratory infection

Klebsiella pneumoniae ST258 is a human pathogen associated with poor outcomes worldwide. We identify a member of the acyltransferase superfamily 3 (atf3), enriched within the ST258 clade, that provides a major competitive advantage for the proliferation of these organisms in vivo. Comparison of a wild-type ST258 strain (KP35) and a Δatf3 isogenic mutant generated by CRISPR-Cas9 targeting reveals greater NADH:ubiquinone oxidoreductase transcription and ATP generation, fueled by increased glycolysis. The acquisition of atf3 induces changes in the bacterial acetylome, promoting lysine acetylation of multiple proteins involved in central metabolism, specifically Zwf (glucose-6 phosphate dehydrogenase). The atf3-mediated metabolic boost leads to greater consumption of glucose in the host airway and increased bacterial burden in the lung, independent of cytokine levels and immune cell recruitment. Acquisition of this acyltransferase enhances fitness of a K. pneumoniae ST258 isolate and may contribute to the success of this clonal complex as a healthcare-associated pathogen.

Staphylococcus aureus induces an itaconate-dominated immunometabolic response that drives biofilm formation

Staphylococcus aureus is a prominent human pathogen that readily adapts to host immune defenses. Here, we show that, in contrast to Gram-negative pathogens, S. aureus induces a distinct airway immunometabolic response dominated by the release of the electrophilic metabolite, itaconate. The itaconate synthetic enzyme, IRG1, is activated by host mitochondrial stress, which is induced by staphylococcal glycolysis. Itaconate inhibits S. aureus glycolysis and selects for strains that re-direct carbon flux to fuel extracellular polysaccharide (EPS) synthesis and biofilm formation. Itaconate-adapted strains, as illustrated by S. aureus isolates from chronic airway infection, exhibit decreased glycolytic activity, high EPS production, and proficient biofilm formation even before itaconate stimulation. S. aureus thus adapts to the itaconate-dominated immunometabolic response by producing biofilms, which are associated with chronic infection of the human airway.

The Salmonella Effector SseK3 Targets Small Rab GTPases

During infection, Salmonella species inject multiple type III secretion system (T3SS) effector proteins into host cells that mediate invasion and subsequent intracellular replication. At early stages of infection, Salmonella exploits key regulators of host intracellular vesicle transport, including the small GTPases Rab5 and Rab7, to subvert host endocytic vesicle trafficking and establish the Salmonella-containing vacuole (SCV). At later stages of intracellular replication, interactions of the SCV with Rab GTPases are less well defined. Here we report that Rab1, Rab5, and Rab11 are modified at later stages of Salmonella infection by SseK3, an arginine N-acetylglucosamine (GlcNAc) transferase effector translocated via the Salmonella pathogenicity island 2 (SPI-2) type III secretion system. SseK3 modified arginines at positions 74, 82, and 111 within Rab1 and this modification occurred independently of Rab1 nucleotide binding. SseK3 exhibited Golgi localization that was independent of its glycosyltransferase activity but Arg-GlcNAc transferase activity was required for inhibition of alkaline phosphatase secretion in transfected cells. While SseK3 had a modest effect on SEAP secretion during infection of HeLa229 cells, inhibition of IL-1 and GM-CSF cytokine secretion was only observed upon over-expression of SseK3 during infection of RAW264.7 cells. Our results suggest that, in addition to targeting death receptor signaling, SseK3 may contribute to Salmonella infection by interfering with the activity of key Rab GTPases.

Pseudomonas aeruginosa Utilizes Host-Derived Itaconate to Redirect Its Metabolism to Promote Biofilm Formation

The bacterium Pseudomonas aeruginosa is especially pathogenic, often being associated with intractable pneumonia and high mortality. How P. aeruginosa avoids immune clearance and persists in the inflamed human airway remains poorly understood. In this study, we show that P. aeruginosa can exploit the host immune response to maintain infection. Notably, unlike other opportunistic bacteria, we found that P. aeruginosa alters its metabolic and immunostimulatory properties in response to itaconate, an abundant host-derived immunometabolite in the infected lung. Itaconate induces bacterial membrane stress, resulting in downregulation of lipopolysaccharides (LPS) and upregulation of extracellular polysaccharides (EPS). These itaconate-adapted P. aeruginosa accumulate lptD mutations, which favor itaconate assimilation and biofilm formation. EPS, in turn, induces itaconate production by myeloid cells, both in the airway and systemically, skewing the host immune response to one permissive of chronic infection. Thus, the metabolic versatility of P. aeruginosa needs to be taken into account when designing therapies.

Metabolic Adaptation in Methicillin-Resistant Staphylococcus aureus Pneumonia

Methicillin-resistant Staphylococcus aureus (MRSA) is a versatile human pathogen that is associated with diverse types of infections ranging from benign colonization to sepsis. We postulated that MRSA must undergo specific genotypic and phenotypic changes to cause chronic pulmonary disease. We investigated how MRSA adapts to the human airway to establish chronic infection, as occurs during cystic fibrosis (CF). MRSA isolates from patients with CF that were collected over a 4-year period were analyzed by whole-genome sequencing, transcriptional analysis, and metabolic studies. Persistent MRSA infection was associated with staphylococcal metabolic adaptation, but not changes in immunogenicity. Adaptation was characterized by selective use of the tricarboxylic acid cycle cycle and generation of biofilm, a means of limiting oxidant stress. Increased transcription of specific metabolic genes was conserved in all host-adapted strains, most notably a 10,000-fold increase in fumC, which catalyzes the interconversion of fumarate and malate. Elevated fumarate levels promoted in vitro biofilm production in clinical isolates. Host-adapted strains preferred to assimilate glucose polymers and pyruvate, which can be metabolized to generate N-acetylglucosamine polymers that comprise biofilm. MRSA undergoes substantial metabolic adaptation to the human airway to cause chronic pulmonary infection, and selected metabolites may be useful therapeutically to inhibit infection.

Pulmonary Pathogens Adapt to Immune Signaling Metabolites in the Airway

A limited number of pulmonary pathogens are able to evade normal mucosal defenses to establish acute infection and then adapt to cause chronic pneumonias. Pathogens, such as Pseudomonas aeruginosa or Staphylococcus aureus, are typically associated with infection in patients with underlying pulmonary disease or damage, such as cystic fibrosis (CF) or chronic obstructive pulmonary disease (COPD). To establish infection, bacteria express a well-defined set of so-called virulence factors that facilitate colonization and activate an immune response, gene products that have been identified in murine models. Less well-understood are the adaptive changes that occur over time in vivo, enabling the organisms to evade innate and adaptive immune clearance mechanisms. These colonizers proliferate, generating a population sufficient to provide selection for mutants, such as small colony variants and mucoid variants, that are optimized for long term infection. Such host-adapted strains have evolved in response to selective pressure such as antibiotics and the recruitment of phagocytes at sites of infection and their release of signaling metabolites (e.g., succinate). These metabolites can potentially function as substrates for bacterial growth and but also generate oxidant stress. Whole genome sequencing and quantified expression of selected genes have helped to explain how P. aeruginosa and S. aureus adapt to the presence of these metabolites over the course of in vivo infection. The serial isolation of clonally related strains from patients with cystic fibrosis has provided the opportunity to identify bacterial metabolic pathways that are altered under this immune pressure, such as the anti-oxidant glyoxylate and pentose phosphate pathways, routes contributing to the generation of biofilms. These metabolic pathways and biofilm itself enable the organisms to dissipate oxidant stress, while providing protection from phagocytosis. Stimulation of host immune signaling metabolites by these pathogens drives bacterial adaptation and promotes their persistence in the airways. The inherent metabolic flexibility of P. aeruginosa and S. aureus is a major factor in their success as pulmonary pathogens.

Strains of Staphylococcus aureus that Colonize and Infect Skin Harbor Mutations in Metabolic Genes

Staphylococcus aureus is the most common cause of skin and soft tissue infections, yet the bacterial genetic changes associated with adaptation to human skin are not well characterized. S. aureus strains isolated from patients with chronic skin colonization and intermittent infection were used to determine the staphylococcal genotypes or phenotypes associated with adaptation to human skin. We demonstrate that polymorphisms in metabolic genes, particularly those involved in the tricarboxylic acid cycle, the fumarate-succinate axis, and the generation of terminal electron transporters, are unexpectedly common. These skin-adapted strains activated glycolysis and hypoxia-inducible factor-1α, interleukin (IL)-1β, and IL-18 release from keratinocytes and promoted dermatopathology equivalent to a methicillin-resistant Staphylococcus aureus USA300 control in a murine model of infection. However, in contrast to USA300, a skin-adapted isolate failed to generate protection from a secondary infectious challenge. Within the context of human skin, there appears to be selection for S. aureus metabolic adaptive changes that promote glycolysis and maintain pathogenicity.

Salmonella Effectors SseK1 and SseK3 Target Death Domain Proteins in the TNF and TRAIL Signaling Pathways

Strains of Salmonella utilize two distinct type three secretion systems to deliver effector proteins directly into host cells. The Salmonella effectors SseK1 and SseK3 are arginine glycosyltransferases that modify mammalian death domain containing proteins with N-acetyl glucosamine (GlcNAc) when overexpressed ectopically or as recombinant protein fusions. Here, we combined Arg-GlcNAc glycopeptide immunoprecipitation and mass spectrometry to identify host proteins GlcNAcylated by endogenous levels of SseK1 and SseK3 during Salmonella infection. We observed that SseK1 modified the mammalian signaling protein TRADD, but not FADD as previously reported. Overexpression of SseK1 greatly broadened substrate specificity, whereas ectopic co-expression of SseK1 and TRADD increased the range of modified arginine residues within the death domain of TRADD. In contrast, endogenous levels of SseK3 resulted in modification of the death domains of receptors of the mammalian TNF superfamily, TNFR1 and TRAILR, at residues Arg376 and Arg293 respectively. Structural studies on SseK3 showed that the enzyme displays a classic GT-A glycosyltransferase fold and binds UDP-GlcNAc in a narrow and deep cleft with the GlcNAc facing the surface. Together our data suggest that salmonellae carrying sseK1 and sseK3 employ the glycosyltransferase effectors to antagonise different components of death receptor signaling.

Metabolic Stress Drives Keratinocyte Defenses against Staphylococcus aureus Infection

Human skin is commonly colonized and infected by Staphylococcus aureus. Exactly how these organisms are sensed by keratinocytes has not been clearly delineated. Using a combination of metabolic and transcriptomic methodologies, we found that S. aureus infection is sensed as a metabolic stress by the hypoxic keratinocytes. This induces HIF1α signaling, which promotes IL-1β production and stimulates aerobic glycolysis to meet the metabolic requirements of infection. We demonstrate that staphylococci capable of glycolysis, including WT and agr mutants, readily induce HIF1α responses. In contrast, Δpyk glycolytic mutants fail to compete with keratinocytes for their metabolic needs. Suppression of glycolysis using 2-DG blocked keratinocyte production of IL-1β in vitro and significantly exacerbated the S. aureus cutaneous infection in a murine model. Our data suggest that S. aureus impose a metabolic stress on keratinocytes that initiates signaling necessary to promote both glycolysis and the proinflammatory response to infection.

EspL is a bacterial cysteine protease effector that cleaves RHIM proteins to block necroptosis and inflammation

Cell death signalling pathways contribute to tissue homeostasis and provide innate protection from infection. Adaptor proteins such as receptor-interacting serine/threonine-protein kinase 1 (RIPK1), receptor-interacting serine/threonine-protein kinase 3 (RIPK3), TIR-domain-containing adapter-inducing interferon-β (TRIF) and Z-DNA-binding protein 1 (ZBP1)/DNA-dependent activator of IFN-regulatory factors (DAI) that contain receptor-interacting protein (RIP) homotypic interaction motifs (RHIM) play a key role in cell death and inflammatory signalling^1-3. RHIM-dependent interactions help drive a caspase-independent form of cell death termed necroptosis^4,5. Here, we report that the bacterial pathogen enteropathogenic Escherichia coli (EPEC) uses the type III secretion system (T3SS) effector EspL to degrade the RHIM-containing proteins RIPK1, RIPK3, TRIF and ZBP1/DAI during infection. This requires a previously unrecognized tripartite cysteine protease motif in EspL (Cys47, His131, Asp153) that cleaves within the RHIM of these proteins. Bacterial infection and/or ectopic expression of EspL leads to rapid inactivation of RIPK1, RIPK3, TRIF and ZBP1/DAI and inhibition of tumour necrosis factor (TNF), lipopolysaccharide or polyinosinic:polycytidylic acid (poly(I:C))-induced necroptosis and inflammatory signalling. Furthermore, EPEC infection inhibits TNF-induced phosphorylation and plasma membrane localization of mixed lineage kinase domain-like pseudokinase (MLKL). In vivo, EspL cysteine protease activity contributes to persistent colonization of mice by the EPEC-like mouse pathogen Citrobacter rodentium. The activity of EspL defines a family of T3SS cysteine protease effectors found in a range of bacteria and reveals a mechanism by which gastrointestinal pathogens directly target RHIM-dependent inflammatory and necroptotic signalling pathways.

SseK3 Is a Salmonella Effector That Binds TRIM32 and Modulates the Host's NF-κB Signalling Activity

Salmonella Typhimurium employs an array of type III secretion system effectors that facilitate intracellular survival and replication during infection. The Salmonella effector SseK3 was originally identified due to amino acid sequence similarity with NleB; an effector secreted by EPEC/EHEC that possesses N-acetylglucoasmine (GlcNAc) transferase activity and modifies death domain containing proteins to block extrinsic apoptosis. In this study, immunoprecipitation of SseK3 defined a novel molecular interaction between SseK3 and the host protein, TRIM32, an E3 ubiquitin ligase. The conserved DxD motif within SseK3, which is essential for the GlcNAc transferase activity of NleB, was required for TRIM32 binding and for the capacity of SseK3 to suppress TNF-stimulated activation of NF-κB pathway. However, we did not detect GlcNAc modification of TRIM32 by SseK3, nor did the SseK3-TRIM32 interaction impact on TRIM32 ubiquitination that is associated with its activation. In addition, lack of sseK3 in Salmonella had no effect on production of the NF-κB dependent cytokine, IL-8, in HeLa cells even though TRIM32 knockdown suppressed TNF-induced NF-κB activity. Ectopically expressed SseK3 partially co-localises with TRIM32 at the trans-Golgi network, but SseK3 is not recruited to Salmonella induced vacuoles or Salmonella induced filaments during Salmonella infection. Our study has identified a novel effector-host protein interaction and suggests that SseK3 may influence NF-κB activity. However, the lack of GlcNAc modification of TRIM32 suggests that SseK3 has further, as yet unidentified, host targets.

Substrate recognition by the zinc metalloprotease effector NleC from enteropathogenic Escherichia coli

Upon infection of epithelial cells, enteropathogenic Escherichia coli suppresses host cell inflammatory signalling in a type III secretion system (T3SS) dependent manner. Two key T3SS effector proteins involved in this response are NleE and NleC. NleC is a zinc metalloprotease effector that degrades the p65 subunit of NF-κB. Although the site of p65 cleavage by NleC is now well described, other areas of interaction have not been precisely defined. Here we constructed overlapping truncations of p65 to identify regions required for NleC cleavage. We determined that NleC cleaved both p65 and p50 within the Rel homology domain (RHD) and that two motifs, E22IIE25 and P177VLS180 , within the RHD of p65 were important for recognition and binding by NleC. Alanine substitution of one or both of these motifs protected p65 from binding and degradation by NleC. The E22IIE25 and P177VLS180 motifs were located within the structurally distinct N-terminal subdomain of the RHD involved in DNA binding by p65 on adjacent, parallel strands. Although these motifs have not been recognized previously, both were needed for the correct localization and function of p65. In summary, this work has identified two regions of p65 within the RHD needed for binding and cleavage by NleC and provides further insight into the molecular basis of substrate recognition by a T3SS effector.

The cell death response to enteropathogenic Escherichia coli infection

Given the critical roles of inflammation and programmed cell death in fighting infection, it is not surprising that many bacterial pathogens have evolved strategies to inactivate these defences. The causative agent of infant diarrhoea, enteropathogenic Escherichia coli (EPEC), is an extracellular, intestinal pathogen that blocks both inflammation and programmed cell death. EPEC attaches to enterocytes, remains in the gut lumen and utilizes a type III secretion system (T3SS) to inject multiple virulence effector proteins directly into the infected cell, many of which subvert host antimicrobial processes through the disruption of signalling pathways. Recently, T3SS effector proteins from EPEC have been identified that inhibit death receptor-induced apoptosis. Here we review the mechanisms used by EPEC T3SS effectors to manipulate apoptosis and promote host cell survival and discuss the role of these activities during infection.